Python Unicode And Bytes Tutorial | Unicode And Bytes In Python

Python Unicode and bytes are two different parameters used in encoding

Parameter Details
encoding The encoding to use, e.g. , , etc…
'ascii' 'utf8'
The errors mode, e.g. to replace bad characters with question marks, to ignore
errors 'replace' 'ignore'
bad characters, etc…

Python Unicode and bytes: Encoding/decoding error handling

.encode and .decode both have error modes.

The default is ‘strict’, which raises exceptions on error. Other modes are more forgiving.


"£13.55".encode('ascii', errors='replace') b'?13.55'
"£13.55".encode('ascii', errors='ignore') b'13.55'
"£13.55".encode('ascii', errors='namereplace') b'\N{POUND SIGN}13.55'
"£13.55".encode('ascii', errors='xmlcharrefreplace') b'£13.55'
"£13.55".encode('ascii', errors='backslashreplace') b'\xa313.55'


b = "£13.55".encode('utf8')
b.decode('ascii', errors='replace') '��13.55'
b.decode('ascii', errors='ignore') '13.55'
b.decode('ascii', errors='backslashreplace') '\xc2\xa313.55'


It is clear from the above that it is vital to keep your encodings straight when dealing with unicode and bytes.

File I/O

Files opened in a non-binary mode (e.g. ‘r’ or ‘w’) deal with strings. The default encoding is ‘utf8’.

open(fn, mode='r') # opens file for reading in utf8
open(fn, mode='r', encoding='utf16') # opens file for reading utf16
ERROR: cannot write bytes when a string is expected: open("foo.txt", "w").write(b"foo")

Files opened in a binary mode (e.g. ‘rb’ or ‘wb’) deal with bytes. No encoding argument can be specified as there is no encoding.

open(fn, mode='wb') # open file for writing bytes
ERROR: cannot write string when bytes is expected: open(fn, mode='wb').write("hi")

Python Unicode and bytes: Basics

In Python 3 str is the type for unicode-enabled strings, while bytes is the type for sequences of raw bytes.

type("f") == type(u"f") # True,
type(b"f") #
In Python 2 a casual string was a sequence of raw bytes by default and the unicode string was every string with "u" prefix.
type("f") == type(b"f") # True,
type(u"f") #

Unicode to bytes

Unicode strings can be converted to bytes with .encode(encoding).

Python 3

"£13.55".encode('utf8') b'\xc2\xa313.55'

Python 2

in py2 the default console encoding is sys.getdefaultencoding() == ‘ascii’ and not utf-8 as in py3, therefore printing it as in the previous example is not directly possible.

print type(u"£13.55".encode('utf8'))
print u"£13.55".encode('utf8') SyntaxError: Non-ASCII character '\xc2' in…
with encoding set inside a file
-- coding: utf-8 --
print u"£13.55".encode('utf8') ┬ú13.55

If the encoding can’t handle the string, a UnicodeEncodeError is raised:

"£13.55".encode('ascii') Traceback (most recent call last):
File "", line 1, in
UnicodeEncodeError: 'ascii' codec can't encode character '\xa3' in position 0: ordinal not in range(128)

Bytes to unicode

Bytes can be converted to unicode strings with .decode(encoding).

A sequence of bytes can only be converted into a unicode string via the appropriate encoding!


If the encoding can’t handle the string, a UnicodeDecodeError is raised:

b'\xc2\xa313.55'.decode('utf16') Traceback (most recent call last):
File "", line 1, in
File "/Users/csaftoiu/csaftoiu-github/yahoo-groups-backup/.virtualenv/bin/../lib/python3.5/encodings/", line 16, in decode
return codecs.utf_16_decode(input, errors, True)
UnicodeDecodeError: 'utf-16-le' codec can't decode byte 0x35 in position 6: truncated data


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